spl: Convert boot_device into a struct

At present some spl_xxx_load_image() functions take a parameter and some
don't. Of those that do, most take an integer but one takes a string.

Convert this parameter into a struct so that we can pass all functions the
same thing. This will allow us to use a common function signature.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/arm/mach-uniphier/boot-mode/spl_board.c b/arch/arm/mach-uniphier/boot-mode/spl_board.c
index 63ab41c..4eadc2f 100644
--- a/arch/arm/mach-uniphier/boot-mode/spl_board.c
+++ b/arch/arm/mach-uniphier/boot-mode/spl_board.c
@@ -65,7 +65,7 @@
 	return 0;
 }
 
-int spl_board_load_image(void)
+int spl_board_load_image(struct spl_boot_device *bootdev)
 {
 	int (*send_cmd)(u32 cmd, u32 arg);
 	int (*card_blockaddr)(u32 rca);